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Abstract

Co₃O₄ with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co³⁺ is considered to be the active site, while tetrahedrally coordinated Co²⁺ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H₂ reduction of nanocast Co₃O₄ at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co²⁺ octahedrally coordinated by lattice oxygen in Fm3̅m symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co²⁺ to Co³⁺ causes the high activity. Other mesoporous spinels (CuCo₂O₄, CoCr₂O₄, and CoFe₂O₄) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co³⁺ highly active but also the octahedrally coordinated Co²⁺ species in CoFe₂O₄ with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co²⁺ species is easily oxidized and shows high catalytic activity for CO oxidation.